The present invention concerns a method of removing the hull from corn kernels wherein the method involves exposing the corn kernels to ammonia (e.g., gas-phase anhydrous) under conditions effective to remove the hull from corn kernels.
Current technology for the large-scale conversion of corn (maize) to value-added products involves either a wet milling process or a dry milling process. The corn wet milling process begins by steeping (soaking) the corn kernels in water which contains added SO2. Steeping may take up to 48 hours in large expensive steep tanks. Steeping softens the corn so that coarse grinding will release the intact germ which can be separated and processed to recover the oil; further grinding then permits separation of the remaining components (e.g., fiber, protein and starch). However, building a corn wet mill requires a large capital investment. In contrast, although less capital intensive, the dry milling process to make food products or fuel ethanol suffers from low co-product value.
The wet milling process and the dry milling process can be modified by removing the hull (pericarp) of the corn kernel as the first processing step. In a modification of the dry-grind process known as Quick-Germ, the germ can be recovered after 12 hours soaking to improve the co-product credits (Singh, V., et al., Cereal Chemistry, 73(6): 716-720 (1996)). Diffusion of water into the kernel during steeping or soaking is slow because the hull (pericarp) covering the kernel forms a waterproof barrier, the time required for steeping in the wet milling process or soaking in the Quick-Germ process can be reduced if the pericarp is removed. Alkali debranning of grains is usually done with a caustic soda (NaOH) solution, which loosens the hulls, so that mechanical equipment may remove and separate the hulls from the grain. (Du, L., et al., Cereal Chemistry, 76(5): 811-815 (1999)); Morgan, A. I., et al., Food Technology, pp. 40-43 (August 1964)). However, treatment with alkali has certain disadvantages: When treating with caustic solution, most of the alkali remains external to the corn. Although the solution can be reused a number of times, it eventually must be discarded and replaced with fresh solution. The consumption of sodium hydroxide is typically 2 to 3% of treated grain. Disposal of the waste can be very expensive.
We have discovered a superior method of removing the pericarp by exposing the grain to ammonia (e.g., gas-phase anhydrous) which diffuses into the kernel more easily than liquid caustic and dissolves in the moisture that constitutes approximately 15% of dry corn. Because the resulting strong base solution will be entirely inside the kernel, the time, temperature and amount of base needed is less than with caustic solution. Ammonia is also less expensive than caustic. Residual ammonia in the corn remaining after debranning and germ recovery can supply the nitrogen requirement for yeast to ferment corn to ethanol. This ammonia treatment loosens the pericarp from whole corn using no more ammonia than needed to supply the nitrogen requirement for yeast fermentation. Ammonia treatment also helps to separate the starch and protein from fiber in the remaining corn.
The present invention concerns a method of removing the hull from corn kernels wherein the method involves exposing the corn kernels to ammonia (e.g., gas-phase anhydrous) under conditions effective to remove the hull from corn kernels.